Generally, an automatic transmission to be mounted in a vehicle is provided with a plurality of hydraulic frictional engagement elements such as planetary gear sets, clutches, and brakes; and is configured to attain a plurality of forward gear positions and a usually one reverse gear position by selectively engaging these frictional engagement elements by hydraulic control for switching a power transmission path via each of the planetary gear sets. In recent years, however, the number of gear positions are tended to increase for the purpose of improving the traveling performance, improving the fuel economy of an engine, reducing electric power consumption of a drive motor, or the like.
For instance, Patent Literature 1 discloses, as illustrated in FIG. 11, an automatic transmission provided with three planetary gear sets PG11, PG12, PG13, four clutches CL11, CL12, CL13, CL14, and two brakes BR11, BR12, wherein two of these clutches and brakes are engaged to attain nine forward gear positions and one reverse gear position.
The automatic transmission described in Patent Literature 1 is a transversely disposed automatic transmission for use in an FF vehicle (a front-engine front-drive vehicle) or the like, and is mounted in a posture in which the axis thereof is aligned with the vehicle width direction. Power from a power source such as an engine or a motor is input from an input shaft A disposed on the axis, and power via a transmission mechanism constituted by the planetary gear sets, the clutches, and the like is output from an output portion B formed on a carrier C13 of the third planetary gear set PG13 to an unillustrated shaft disposed in parallel to the input shaft A via a gear or the like.
There is a case, in which a transversely disposed automatic transmission is changed to a longitudinally disposed automatic transmission usable for an FR vehicle (a front-engine rear-drive vehicle), namely, an automatic transmission, in which an input shaft and an output shaft are coaxially disposed in the vehicle front-rear direction without changing the configuration of the transmission mechanism. However, in the automatic transmission disclosed in Patent Literature 1, the output portion B is disposed between the third planetary gear set PG13 and the second brake BR12, and a coupling member C for coupling a sun gear S13 of the third planetary gear set PG13 and the second brake BR12 exists on the inner peripheral side of the output portion B. Therefore, in the aforementioned configuration, it may not be possible to couple the output portion B to the output shaft disposed coaxially with the input shaft A.
On the other hand, as illustrated in FIG. 12, if the third planetary gear set PG13 is configured as a double sun gear type planetary gear set in which a sun gear is split into axially aligned first and second sun gears S131 and S132, and an output portion B′ passing between the first and second sun gears S131 and S132 and extending radially inwardly is formed on the carrier C13, it is possible to couple the output portion B′ to an output shaft D disposed coaxially with the input shaft. This makes it possible to change a transversely disposed automatic transmission to a longitudinally disposed automatic transmission without greatly changing the configuration of the automatic transmission.
However, in the configuration illustrated in FIG. 12, the output shaft D passes through the split sun gear S132 of the third planetary gear set PG13. In this case, it is necessary to set the diameter of the output shaft D large, as compared with the input shaft A in view of the necessity of transmitting a torque, which is larger than the input torque by the amount corresponding to a maximum speed reduction ratio. This may make the diameter of the sun gear S132 large, and the overall diameter of the third planetary gear PG13 may increase accordingly.
An increase in the diameter of the third planetary gear PG13 as described above may increase the diameter of an end portion of an automatic transmission on the output side. In particular, when an automatic transmission is disposed below a floor at the middle of a vehicle compartment front portion in the vehicle width direction of an FR vehicle, the width of a driver's seat and a passenger's seat in a vehicle compartment is limited. This may deteriorate the seating comfort. Note that the aforementioned inconvenience is an inconvenience common to a longitudinally disposed automatic transmission, in which a planetary gear set is disposed on an output shaft whose diameter is relatively large, regardless of the number of gear positions or other configuration.